The invention relates to a screening machine with a three-dimensional screening motion, having a carrier for at least one screen plate, wherein the screen carrier that is movably arranged relative to a housing and is driven by an electromagnet is supported at the housing by spring means.
A screening machine of this type is described in the company brochure "Retsch-Labor Siebmaschine, Vibro, 006/1985" (Retsch-Laboratory Screening machine, Vibro, 006/1985). In this screening machine the screen carrier includes a three-armed spider with its spider arms being supported on springs arranged at the housing so that it is movable relative to the housing. The springs, in a tangential path, are arranged in an inclined position relative to the vertical axis of the screening machine. An electromagnet arranged below the screen carrier, generates, by acting on the spider of the screen carrier, a vertical amplitude of oscillation and this vertical motion of the screen carrier is converted via the motion along the axis of the inclined springs into a three-dimensional motion of the screen plates carried by the screen carrier.
The accuracy of a grain size analysis to be undertaken with a screening machine of the aforementioned kind is determined by adhering to or knowing the following parameters: correct mesh size of the screen plates according to DIN (German Industrial Standard); analysis time; frequency of the screen motion; amplitude of oscillation of the screen motion. In order to guarantee the comparability of various analyses these parameters must be defined for the analyses carried out and must be reproducible for further analyses in order to arrive at a reliable result of analysis.
However, such a screening machine disadvantageously does not meet these requirements, in particular with respect to the determination of the amplitude of oscillation. The amplitude of oscillation may only be determined by means of a scale read according to the principle of optical illusion. For this purpose a triangle extending transverse to the oscillating axis is attached to the screen carrier, the sides of which, when the screen machine is in operation, apparently converge at one point. This point, on a correspondingly determined scale, should correspond to the dimensions of the amplitude of oscillation to be detected. It will be understood that this type of determining the amplitude of oscillation must be insufficient with regard to its accuracy, because the optical reading of a measuring point is dependent on the observation and the assessment of the operator. A further disadvantage is that with various assemblies of screen plates in the screening machine, a desired or previously adjustable amplitude of oscillation for each assembly is only obtainable by manually readjusting the driving energy of the electromagnet, but accordingly the desired measuring point cannot be adjusted with the desired accuracy due to the manual regulation of the electromagnetic drive device.
The object of the present invention is therefore to improve the screening machine of the aforementioned kind, so that the amplitude of oscillation during an analysis procedure may be read with sufficient accuracy. A further aspect of the present invention is that a certain or predetermined amplitude of oscillation may be obtained during operation of the screening machine, irrespective of the selected assembly of screen plates in the screening machine.